The Big Dipper and the Cross

The Big Dipper and the Cross
The Big Dipper and other objects this time of year as seen from Toronto at 10:30 p.m. local time. The view will be different from other locations.

As soon as darkness falls these evenings, step outside and look skyward.  What is the most prominent and easiest star pattern to recognize? 

If you live in the Northern Hemisphere you only need to look overhead and toward the north where you will find the seven bright stars that comprise the famous Big Dipper. [Map]

Interestingly, in other parts of the world, these seven stars are known not as a Dipper, but as some sort of a wagon.  In Ireland, for instance, it was recognized as "King David's Chariot," from one of that island's early kings; in France, it was the "Great Chariot."  Another popular name was Charles's Wain (a wain being a large open farm wagon).  And in the British Isles, these seven stars are known widely as "The Plough."

For most sky gazers, the Big Dipper (or Plough) is probably the most important group of stars in the sky.  For anyone in the latitude of New York (41 degrees North) or points northward, it never goes below the horizon.  It is one of the most recognizable patterns in the sky and thus one of the easiest for the novice to find. 

Southern Hemisphere

But for those who live in the Southern Hemisphere, it is not the Big Dipper that people choose as their guide to the night sky - but rather, it's the constellation known as Crux, the Southern Cross. [Map]

Those south of the equator (where it is now mid-autumn), need only cast a glance toward the south where they'll see the distinctive shape of the Cross hanging well up in the sky.  To some, it looks more like a kite, though the Cross is clearly outlined by four bright stars, two of which, Acrux and Becrux are of the first magnitude.  From top to bottom, Crux measures just 6 degrees - only a little taller than the distance between the Pointer stars of the Big Dipper. 

In fact, the Southern Cross is the smallest (in area) of all the constellations. Like the Big Dipper, whose Pointer stars point toward Polaris, the North Star, the Southern Cross indicates the location of the South Pole of the sky and as such is often utilized by navigators.  The longer bar of the Cross points almost exactly toward the South Pole of the sky which some aviators and navigators have named the "south polar pit" because, unfortunately, it is not marked by any bright star. 

Amerigo Vespucci

It is thought that Amerigo Vespucci was the first of the European voyagers to see the "Four Stars," as he called them, while on his third voyage in 1501.  But actually, Crux was plainly visible everywhere in the United States some 5,000-years ago, as well as in ancient Greece and Babylonia.

According to Richard Hinckley Allen (1838-1908), an expert in stellar nomenclature, the Southern Cross was last seen on the horizon of Jerusalem about the time that Christ was crucified.  Thanks to precession - an oscillating motion of the Earth's axis - over the centuries, the Cross ended up gradually getting shifted out of view well to the south.

Immediately to the south and east of the Cross is a pear-shaped, inky spot, about as large as the Cross itself, looking like a great black hole in the midst of the Milky Way.  When Sir John Herschel first saw it from the Cape of Good Hope, South Africa in 1835, it is said that he wrote his aunt, Caroline about this "hole in the sky."  Indeed, few stars are seen within this hole and it soon became popularly known as the "Coalsack" which initially was thought to be some sort of window into outer space. 

Today we know that the celebrated Coalsack is really a great cloud of gas and dust that absorbs the light of the stars that must lie beyond it.

Never seen one of them?

There are probably a number of you who have never seen either the Big Dipper or the Southern Cross and might wonder about how far they might have to travel in order to get a view of them.  Coincidentally, at this time of the year, both are attaining their highest positions in the sky at the same time: around 10-p.m. local daylight time in mid-May.  To see Crux, one must go at least as far south as latitude 25 degrees North. 

That means heading to the Florida Keys in the continental United States, where you'll see it just lifting fully above the southern horizon.  A slightly better view is afforded to those living in Hawaii, where the Cross appears a few degrees higher.

So far as seeing the Big Dipper, you must go north of latitude 25 degrees South to see it in its entirety.  Across the northern half of Australia, for instance, you can now just see the upside-down Dipper virtually scraping the northern horizon about an hour or two after sundown. 

In fact, it's just the opposite effect as opposed to those who live in north temperate latitudes (like New York), whose inhabitants see the Dipper at a similar altitude above the northern horizon on early evenings in late November or early December - except the Dipper appears right-side up!

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).

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Joe Rao
Skywatching Columnist

Joe Rao is's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.